1. Field of the Invention
The present invention relates to a device for detection of wetting of a pane of glass, or windshield, and, more particularly, to a device for detecting wetting of a pane of glass, or windshield, including a rain sensor with a measurement circuit comprising a light-radiating transmitter and a receiver for the light from the light-radiating transmitter and an evaluating device connected with the receiver to evaluate light impinging on the receiver.
2. Prior Art
A device having an optical rain sensor and an evaluating unit is described in a previous German patent application (number 196 21 627). The rain sensor detects the wetting of a pane of glass or windshield by means of a light-radiating transmitter and a light receiver which responds to the radiated light and delivers a sensor signal to the evaluating unit. The windshield, as a measurement path of the rain sensor, is located in the optical beam path.
A measurement circuit in the rain sensor has a capacitor as an integration stage for detecting the amount of light striking the receiver and a comparator stage which delivers a trigger signal to the evaluating unit when the quantity of light integrated after a certain point in time exceeds a given threshold within a time interval. For this purpose, the total amount of light formed by the light of the transmitter and possibly by extraneous light and the amount of extraneous light detected when the transmitter is switched off are measured in separate time intervals. The evaluating unit has a timer which generates a count corresponding to the integration period preceding the trigger signal of the comparator stage.
During a first time interval for total light measurement, the transmitter of the rain sensor is switched on and radiates light onto the receiver over the pane of glass used as measurement path. In so doing, extraneous light from the surroundings also reaches the receiver. A value for the total amount of light is derived from the integration period. In the second time interval for extraneous light measurement, the transmitter of the rain sensor is switched off, so that only extraneous light can reach the receiver. A value for the quantity of extraneous light is derived from this second integration period.
A useful light quantity corresponding to the wetting of the pane of glass is determined from the difference between the total light quantity and the extraneous light quantity. An automatic motor vehicle windshield wiping system, for example, is controlled depending on the useful light quantity.
A disadvantage consists in that a new extraneous light measurement is carried out only when there is a change in the total amount of light and, for this reason, a change in the amount of extraneous light is not detected when the total amount of light remains constant.
A further disadvantage consists in that frequent changes in the total light quantity, e.g., when driving along a shadowy street or through a tunnel, the extraneous light measurement is likewise carried out more frequently and the irregular switch-on and switch-off times of a transmitter diode which are brought about for this reason cause an unwanted irregular fluctuation of the radiation output of the transmitter.
In this connection, it is especially disadvantageous that the extraneous light quantity is evaluated only in connection with the total light quantity and a separate extraneous light evaluation is not carried out. Day-night detection is not carried out in this case.
In addition, there are commercially known rain sensors which, in addition to the transmitter and the receiver for detecting the wetting of the windshield, also have a photocell for day-night detection for controlling the trigger sensitivity of the rain sensor. It is disadvantageous that the photocell, as an additional component of the rain sensor, adds to the expense of producing the sensor.
According to the invention the device for detecting wetting of a pane of glass, especially a windshield of a motor vehicle, includes a measurement circuit comprising a light-radiating transmitter for generating radiated light, a light receiver arranged to receive a portion of the radiated light according to the wetting of the pane of glass or windshield as well as extraneous light and to generate a sensor signal depending on a light amount of that portion of the radiated light and the extraneous light reaching the receiver, an integration stage for integrating the sensor signal from the light receiver and thus to obtain an integrated sensor signal and a comparator for generating a trigger signal when the integrated sensor signal exceeds a predetermined threshold; and an evaluating circuit including a device for turning on the light-radiating transmitter during first predetermined time intervals and for turning off the light-radiating transmitter during second predetermined time intervals for evaluating the extraneous light and for day-night detection; a device for determining an integrated light quantity reaching the light receiver during each time interval when a respective trigger signal is received; a device for adjusting the predetermined threshold of the comparator to a first threshold value during each first predetermined time interval during which the light-radiating transmitter is turned on and to a second threshold value during each second predetermined time interval during which the light-radiating transmitter is turned off for evaluating the extraneous light and for day-night detection and a device for adjusting each second predetermined time interval to a first time interval value for evaluating the extraneous light and to a second time interval value for day-night detection after a predetermined number of extraneous light measurements, the second time interval value being longer than the first time interval value.
The device according to the invention has the advantage that the extraneous light is detected and evaluated for purposes of day-night detection. For this purpose, after a given quantity of extraneous light measurements within a short interval of time, an extraneous light measurement is carried out over a longer time interval or with a lower threshold for accurate detection of small amounts of extraneous light.
It is particularly advantageous that the rain sensor has only one receiver which detects the transmitter light for the wetting of the pane of glass and detects the extraneous light for day-night detection.
It is particularly advantageous that the device can be used for controlling a windshield wiper system and other devices. For example, depending on the wetting of the windshield and/or day-night detection, windshield wipers are controlled, headlights are switched on or off or a sliding roof is actuated.
It is further advantageous that the sensitivity or the switching threshold of the rain sensor or the amplification of the sensor signal can be adjusted depending on the amount of extraneous light.